The present invention relates to a system for regenerating data from Non-Return-to-Zero mode data received via a transmission or recording medium to recover the original diata waveform.
Non-Return-to-Zero coding is a mode of transmission or recording-and-readout in which it is not necessary for the signal to return to zero after each item of transmitted or recorded data. One pulse amplitude level represents digital 1 and another pulse level, which may or may not be of opposite polarity, represents 0. This mode of transmission or recording is advantageous in that it permits storing about twice as much data as can be recorded with the return-to-zero mode. However, the signal transmitted with the Non-Return-to-Zero mode tends to distort in pulse waveform during transmission or recording-readout process so that the received waveform is remarkably different from the original one if the intermediary system has unfavorable transmission or recording-reproducing characteristics.
The received signal is usually applied to an input of a comparator for making a comparison with a predetermined threshold level in response to a timing pulse known as data strobe clock pulse to permit discrimination of the signal between digital 1 or 0. If the setting point of the threshold level should deviate from the correct value, however, the system will erroneously discriminate the digital values. Furthermore, if the data strobe clock pulse is out of phase with correct timing and if the signal is obscured by noise in the system, a similar situation is likely to occur.
One approach to this problem would be to adjust the setting of the comparator threshold at a point intermediate between the maximum and minimum peaks of the transmitted signal. However, a satisfactory result is difficult to obtain if the transmitted waveform is distorted by noise to such an extent that the peak amplitude levels of opposite polarities are not symmetrical with respect to each other. Another approach would be to utilize the peak level of a synchronization pulse or the pedestal of a digital video or audio signal as a reference. However, the circuits available for clamping such voltage levels are far from the ideal for this purpose. Even if an ideal clamping circuit should become available, the threshold level would deviate from the correct setting if the received data contains a series of digital 1's or a series of 0's, or if the signal is reproduced from video tape recorders having different wave response characteristics due to their particular operating performance and particular types of recording medium.